Science oft resembles the federal tax code: the rules are rigid, but they also keep changing. So it has been with the study of neurogenesis, or the creation of neurons in the human brain. Not long ago a hard-and-fast rule held that neurons could neither divide nor emerge from elsewhere. The neurons you were born with, in short, were the ones you took to your grave. That dogma began to change in the 1980s, however, when Fernando Nottebohm of the Rockefeller University discovered that neurons were dividing in the forebrains of canaries. It appeared that neurons divided after all.

Neuroscientists resisted this idea at first. It clashed with all established facts and belief. But slowly the accumulation of data moved minds. Now newborn brain cells are popping up in studies like mushrooms after rain. Scientists have even found that neurogenesis increases after physical exercise--a great relief to baby boomers who fried too many brain cells in college. The latest research shows that new neurons are hatching in the most hallowed of all brain regions, the hippocampus, the seat of declarative and spatial memory. These discoveries raise the prospect that we might learn to manipulate neurogenesis to relieve ailments such as stroke and cognitive decline.